Proteins located in the plasma membrane or surface membranes of target cells are amongst the most prominent, accessible and attractive sites for intervention with small molecule drugs for pharmaceutical and agrochemical purposes. For example, drugs such as ouabain and the cardiac glycosides are effective therapeutics in the treatment of heart disease because of their activity against isoforms of the membrane protein Na+, K+-ATPase of mammalian cells (Schwartz A, et al, 1982).
Individual membrane proteins of interest that are located at the cell surface may be constitutively expressed cellular components found in a host or a pathogenic organism. Alternatively, the expression of these proteins may also be affected by mutation or by interactions between such cells and other organisms. These membrane proteins include transporters, channels, receptors and enzymes plus proteins with structural, regulatory or unknown roles. Various members of these classes of proteins are known to affect the growth, viability, and functional capacity of host organisms, tissues or cells. In particular, several classes of membrane proteins are known to be involved in drug resistance. These include the drug efflux pump proteins which act to increase the efflux of particular drugs, such as antibiotics and other xenobiotics, from the inside of a cell to the outside. This activity lowers the concentration of the drug at the intracellular target site to levels which are no longer effective. Yeast cell expression systems for testing drugs that inhibit drug efflux pump proteins are known. Decottignies et al 1998 describes a number of strains of Saccharomyces cerevisiae in which varying endogenous drug efflux pump proteins (ABC transporter proteins) have been deleted and a further endogenous membrane protein overexpressed in the cell membrane. Such a system also employs the use of regulators which aid in this overexpression. Examples of such regulators are described in Carjaval et al (1997). However, such a system is restrictive in its application as it may be species
specific, ie it may only identify potential drugs useful in inhibiting drug resistance in Saccharomyces cerevisiae. 
As the problem of drug resistance is widely found in all fauna and flora, and not just in yeast, there exists a need to develop a simple in vitro cell based membrane protein expression system for testing potential inhibitors of drug efflux pump proteins, as well as other membrane proteins associated with drug resistance, from different species.
In addition, as the number of potential test compounds, located mainly in compound libraries, is increasing in both size and complexity, there is a need for such a simple in vitro, cell based membrane protein expression system to screen for agonists or antagonists of putative membrane protein drug targets from a broad range of species and which can be adapted for high throughput formats.
It is an object of the present invention to go some way towards providing for these needs and/or to provide the public with a useful choice.